Astrometric Constraints on the Masses of Long-Period Gas Giant Planets in the TRAPPIST-1 Planetary System

TL;DR

This study uses astrometric data from 2011-2016 to refine the distance to TRAPPIST-1 and place upper limits on the masses of potential long-period gas giant planets in its system, finding no evidence for such planets.

Contribution

It provides the first long-term astrometric constraints on the masses of long-period gas giants orbiting TRAPPIST-1, refining distance measurements and setting upper mass limits.

Findings

Refined the distance to TRAPPIST-1 to 12.56 pc.

No evidence found for long-period gas giants more massive than 4.6 M_Jup at 1-year period.

No evidence found for long-period gas giants more massive than 1.6 M_Jup at 5-year period.

Abstract

Transit photometry of the M8V dwarf star TRAPPIST-1 (2MASS J23062928-0502285) has revealed the presence of at least seven planets with masses and radii similar to that of Earth orbiting at distances that might allow liquid water to be present on their surfaces. We have been following TRAPPIST-1 since 2011 with the CAPSCam astrometric camera on the 2.5-m du Pont telescope at the Las Campanas Observatory in Chile. In 2016 we noted that TRAPPIST-1 lies slightly farther away than previously thought, at 12.49 pc, rather than 12.1 pc. Here we examine fifteen epochs of CAPSCam observations of TRAPPIST-1, spanning the five years from 2011 to 2016, and obtain a revised trigonometric distance of $12.56 \pm 0.12$ pc. The astrometric data analysis pipeline shows no evidence for a long-period astrometric wobble of TRAPPIST-1. After proper motion and parallax are removed, residuals at the level of $\pm 1.3$ millarcsec (mas) remain. The amplitude of these residuals constrains the masses of any long-period gas giant planets in the TRAPPIST-1 system: no planet more massive than $\sim 4.6 M_{Jup}$ orbits with a 1 yr period, and no planet more massive than $\sim 1.6 M_{Jup}$ orbits with a 5 yr period. Further refinement of the CAPSCam data analysis pipeline, combined with continued CAPSCam observations, should either detect any long-period planets, or put an even tighter constraint on these mass upper limits.